sub f {
my%s;%r=@_;map{$m{$_}={map{$m{$_}||={};$_=>1}@{$r{$_}}}}keys%r;
reverse sort{($a->{$b})-($b->{$a})}keys%m;
}

##</code><code>##

#!/usr/bin/perl

my @list = (
     a => [ 'b', 'c' ],
     b => [ 'c', 'd' ],
     c => [ 'd' ],
     e => [ 'b', 'a' ],
     f => undef,
);

print join (',', map { "'$_'" } f(@list)),"\n";

##</code><code>##

'd','c','b','a','f','e'